1. Technical Field of the Invention
This invention relates to a process for the improvement in the value of clay--especially kaolinite group minerals.
2. Description of the Prior Art
An increasing demand for high purity materials is part of ongoing technological change. The two most common elements at the surface of the earth, aluminium and silicon, are now in demand as high purity oxides, however these are surprisingly difficult to obtain. This means less than 100 parts per million of impurity, and in some applications of the oxides of aluminium and silicon this figure is substantively less.
There is a simple reason for this, and this is essentially that minerals, by their nature, are not pure compounds. For example, the common oxide of silicon-quartz normally contains a variety of other elements, either subsisting in the silicate lattice or in the channels in the lattice produced by spirals of silicate tetrahedra (SiO.sub.4).sup.4- that are arranged along the vertical crystallographic axis (C). Grinding quartz, in an attempt to liberate or release these impurities has very obvious limitations. It becomes increasingly difficult and costly to grind below a fine particle size. About five micrometres is the limit and even at this size, the energy consumption is considerable.
At a size range of five micrometres, the particle is far beyond the measurement of a molecular cell of quartz, which is slightly less than five angstroms. The comparison between the two measurements is that an angstrom is one ten-thousandth of a micrometre, hence the difficulty of using size reduction to release any entrained elements in the quartz structure.
The solution to this problem of obtaining very pure oxides has been conventional processing e.g. the volatilisation of silicon via halides and then a subsequent hydrolysis. Even here, problems exist, because of volatility of other element halides.
In the case of aluminium a difficulty exists, because of the usual methods of processing bauxite ore that involve a caustic digestion under pressure. The caustic soda (NaOH) used in this digestion is invariably entrained in the hydrated alumina produced. The problem is such that high purity alumina is difficult to obtain free of sodium. This latter element, even at parts per million level can profoundly change the behaviour of alumina at elevated temperatures. This limits use as a refractory material.
A further problem associated with processing of bauxite ore, is that the sodium which is ultimately removed from the alumina is actually in the form of a red mud-like material, which does not harden. It is stored as a pool within a basin which is lined with a black thick plastic. Over time the plastic may develop a small enough aperture to allow the sodium to leach into the soil, thereby creating huge environmental problems.
It has been found that materials that have been leached by the weathering process comprise a high proportion of aluminium and silicon and other elements are in minute amounts.
Kaolinite in the different crystallographic modifications such as kaolinite pM. and kaolinite T. is a common mineral constituent of weathered rocks and it is these minerals that are preferably made use of to ultimately produce relatively pure oxides of alumina and silica, however alumina silicates such as allophane can not be excluded from also being produced.